Elastic promises "open"—delivers proprietary

Okay, reading through other parts of this discussion I've realised what my niggling issue/misunderstanding is here: if the /use/ of the code doesn't provide an avenue for copyright law to bite, how does a person running a service using this SSPL licensed code, assuming they acquired it entirely legally (i.e. they received all the necessary source and so forth), become in any way bound by the terms of the license? They're providing a service, not distributing the code, so how can they end up bound by the terms of a copyright license?

Unless there's some other kind of contract involved, in which case how would that kind of contract obligation be created?

I'm sorry to be asking what are probably dumb questions here, but this really is something that's hard to understand just by trying to read up on it in your spare time . . .